The invention relates to DC to AC conversion methods, and in particular to methods to convert DC power from a solar array into AC power for feeding directly into the electricity grid.
Various methods for DC to AC conversion are known in the art and new methods continue to be invented, as described in U.S. Pat. No. 8,937,822 to current inventor. In the art prior to solid state switching converters, it was known to use rotary converters to convert electrical power of one type into electrical power of another type. A rotary converter of the prior art comprised a motor driven by electrical power of one type at its input, the motor being mechanically connected to drive a generator to produce electrical power of another type at its output. Prior art rotary converters were known in which the motor and generator used separate rotors and field coils, and types were also known in which the motor and generator used the same stator coils and the same rotor, the rotor being wound with a motor winding connected by brushes to a DC input source and a generator winding connected through brushes and a commutator (for DC output) or slip rings (for AC output) to a load. When the input power was at one DC voltage and the output power was at another DC voltage, the rotary converter was also known as a dynamotor. When the input power type was DC and the output power type was AC, the rotary converter was also known as an Inverter. Rotary Inverters were commonly used in aircraft to convert 28 volts DC to 115 volts AC at 400 Hz, but have largely been replaced by solid state inverters in modern aircraft.
The prior art also includes a type of rotary converter for producing 3-phase power from single-phase power: A single-phase induction motor has additional windings from which a second and third phase can be derived. This type of converter is characterized by AC in and AC out that both comprise sinusoidal waveforms at the same frequency.
Neither a motor nor a generator is 100% efficient, therefore the efficiency of a motor-generator combination is the product of the efficiencies of the motor and the generator respectively. For example, if the motor converts DC power to mechanical rotational energy with an efficiency of 75%, and the generator converts rotational energy to AC electrical output power with an efficiency of 80%, then the combined efficiency of DC to AC conversion is 80×75=60%. Low conversion efficiency was thus a characteristic of prior art rotary converters having separate motor and generator sections. Dynamotors and rotary inverters with a common rotor and stator also tended to have low efficiency due to brush friction, brush voltage drop and field power requirements, as well as the fact that having input and output windings on the same rotor limits the gauge of wire that can be used for each.
Rotary inverters have several advantages however; rotary inverters can produce clean, pure sinewave output voltage waveforms; rotary inverters can handle and withstand short periods of high overload due to the inertia of the rotor; rotary inverters can easily produce one, two, three or more output phases and rotary inverters have the potential to be of lower cost than solid state inverters in certain higher power ranges. Other advantages of a rotary inverter in solar energy applications will become apparent upon reading the description herein of the invention. A rotary inverter with improved efficiency, comparable to a solid state DC to AC inverter, can therefore provide an advantageous alternative to purely solid state inverters.